Embodiments of the invention relate to semiconductor manufacturing, and in particular relate to treatment of an inter-layer dielectric (ILD) used to hold electrically conductive features.
In the fabrication of semiconductor devices, substrates are provided and processed to form semiconductor devices. For example, in the fabrication of microchips or dice, semiconductor wafers arc processed for the subsequent formation of individual dice. The initial wafer serves as a substrate to support features such as transistors and conductive metal lines. Processing generally involves depositing and modifying layers of material on the initial wafer for various purposes. For example, an inter-layer dielectric (ILD) may be deposited and pattered to form and electrically isolate conductive metal lines, or traces. Reducing capacitance between the conductive lines is an important goal in the formation of ILD""s.
As semiconductor devices and device features decrease in size, the distance between such conductive lines correspondingly decreases. All other factors remaining constant, this results in a higher capacitance. For example, given parallel conductive lines, capacitance can be determined as   C  =            k      ⁢              xe2x80x83            ⁢      ϵ      ⁢              xe2x80x83            ⁢      A        d  
where (C) is the capacitance. (d) is the distance between the conductive lines, (A) is the area of each conductive line interface, (xcex5) is the permeability of the ILD material, and (k) is the dielectric constant of the ILD material.
It can be seen from the above equation that, all other factors remaining constant, as the distance between lines decreases, the capacitance increases. Unfortunately, as capacitance increases so does signal transmission time, while high frequency capability may be reduced. Other problems such as increased cross-talk can also occur as the capacitance between lines increases.
The dielectric constant, which has no units of measure, is different for different materials. For example, where the dielectric is of a vacuum or air, the dielectric constant is about equal to 1, having no effect on capacitance. However, most ILD materials have a dielectric constant significantly greater than 1. For example, silicon dioxide, a common ILD material, has a dielectric constant generally exceeding 4. Due to the decreasing size of semiconductor features, which decreases the distance between lines, efforts have recently been made to reduce the dielectric constant of the ILD as a means by which to reduce capacitance.
Low dielectric constant materials (i.e., xe2x80x98low kxe2x80x99 materials), such as SiLK(trademark) (a low-k material manufactured by the Dow Chemical Company at the time of this writing) and carbon doped oxides (CDO""s) have been used to form the ILD, thereby reducing capacitance. Unfortunately, such materials are typically weak in mechanical strength when the dielectric constant is below about 2.6. Therefore these materials often deteriorate when exposed to subsequent semiconductor processing. An electron beam may be applied to the ILD in an attempt to rearrange bonds between elements of the ILD, thereby increasing the mechanical strength of the ILD. However, this often leads to the formation of hydroxyl groups within the ILD, which increases the dielectric constant, defeating the purpose of using an otherwise low-k ILD material.